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Sectioning Technique

The technique called section views is a very important aspect of design and documentation, and is used to

• improve the visualization and clarity of new designs,

• clarify multiview drawings,

• reveal interior features of parts, and

• facilitate the dimensioning of drawings.

For mechanical drawings section views are used to reveal interior features of an object that are not easily represented using hidden lines.

Architectural drawings use section views to reveal the interior details of walls, ceilings, and floors.

Sectional drawings are multi-view technical drawings that contain special views of a part or parts, which reveal interior features.

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A primary reason for creating a section view is the elimination of hidden lines.

Section views use a technique that is based on passing an imaginary cutting plane through a part to reveal interior features.

As you can see, the hidden features can be seen after sectioning.

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Section view technique can be used in a) a part drawings:

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b) mechanical assembly drawings:

Assembly section views are typically full or half sections of multiple assembled parts

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c) In solid modeling using CAD software:

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General principles - A sectional view represents the part of an object remaining after a portion is assumed to have been cut and removed.

- The exposed cut surface is then indicated by section lines.

- Hidden features behind the cutting plane are omitted, unless required for dimensioning or for definition of the part.

As you can see here the lines of the edges between surfaces on the rear side of the part, shown as hidden lines on the non-cut

drawing, disappear from the section view.

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Visible surfaces and edges that represent a change of planes or surfaces behind the cutting plane are drawn in a section view:

All hidden lines behind the cutting plane must disappear, but all visible lines should be shown!

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Section lines & symbols Section lines or hatching usually consist of thin parallel lines, as shown below, drawn at an angle of approximately 45° to the

principal edges or axes of the part.

For most purposes the general use symbol of cast iron is used.

When it is desired to indicate differences in materials, for example on assembly drawings involving a variety of materials, other

symbolic section lines may be used:

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If the section lines appear to be parallel, or nearly so, to one of the sides or features of the part, other than 45° angle should be chosen.

Section lines should not run parallel or perpendicular to the visible outline:

The general purpose or cast iron section line is drawn at a 45O angle and spaced 1/16" (1.5mm) to 1/8" (3mm) or more depending on

the size of the drawing.

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In all sections of a single component, section lines should

be similar in direction and spacing, but adjacent parts

should be section-lined in different directions, angles,

or spacing.

Section lines should be thinner than visible lines.

Do not run section lines beyond the visible outlines or

stop them too short.

Section lines should be suitably spaced in

relation to the size of the area covered, and for

large areas it is recommended that section lines

be shown only along the edges.

Thin elements shouldn’t be sectioned.

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Avoid placing dimensions or notes within the section-lined areas. However, where the insertion of dimensions or lettering in sectional areas

is unavoidable, omit the section lines in the area of the note:

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Cutting planes Cutting plane lines which show where the cutting plane passes through the object, represent the edge view of the cutting plane and

are drawn in the view(s) adjacent to the section view:

Here the cutting plane is drawn as an edge in the top view, which is adjacent to the sectioned front view. This is a frontal cutting

plane. Lines of sight should always be directed upwards on the top view for sectioned front view.

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A horizontal cutting plane is one where it is an edge in the front view and the top view is sectioned.

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If the cutting plane appears as an edge in the top and front views and the profile view is sectioned, it is a profile cutting plane.

In the drawing you must show the cutting plane line either on front view or on top view, not on both.

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Two types of lines are acceptable for cutting plane lines in multi-view drawings. It is important to use only one type of cutting

plane line on a single drawing.

Cutting plane lines are thick (0.6 mm) dashed lines, that extend past the edge of the object 6mm (1/4") and have line segments at

each end drawn at 90 degrees and terminated with arrows.

The arrows represent the direction of the line of sight for the section view and they point away from the sectioned view.

The long dash can be lengthened for large section drawings to save time and create a more readable drawing.

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Multiple sections can be done on a single object:

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Cutting planes shall not be shown on sectional views.

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When two or more cutting planes are shown on one drawing,

the cutting planes and section views should be identified by

letters:

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The cutting plane line can often be omitted when it corresponds

to the centre line of the part:

For simple parts the cutting plane line is drawn right across the

part, as in following figure

but for complicated parts, to avoid confusion with other details, it

should terminate close to the outline, as shown below:

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In order to include features that are not in a straight line, the cutting plane may be offset or bent at one or more 90O angles, so as

to include several planes or curved surfaces. It is called offset section and is used for complex parts that have a number of important

features that cannot be sectioned using a straight cutting plane.

The change of plane that occurs when the cutting plane is bent at 900 is not represented with lines in the section view !

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Multiple offset sections used on a single view use labels for identification:

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When the two end planes are parallel (and offset), the section is drawn in true projection unless otherwise indicated, so that

connecting planes are foreshortened and the sectional view shows the part in true projected length, as in the next figure

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When the end planes are not parallel, the non-parallel plane or

planes are revolved into the plane of projection:

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Sometimes it is not necessary to cut the whole part in order to show the section view. Objects that are symmetrical about a centre line

may be drawn having one half as a multiview and the other half in section view.

o Cutting plane line is shown

across the whole part;

o Section plane through center

line of a symmetric part can be

omitted;

o Hidden lines in half sections

are usually omitted

(This drawing is wrong!)

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Again: In case of half sections, if there are hidden feature lines

corresponding to full lines in the sectioned half, such hidden lines

should be omitted from the full view.

In some cases it is more convenient to use a partial

section.

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There is one more type of sections which may be useful

to know.

Revolved section is made by revolving the cross section

view of a feature 90° about an axis of revolution and

superimposing the section view on the orthographic

view.

(A) If the revolved section view does not interfere or

create confusion on the view, then the revolved section

is drawn directly on the view using visible lines.

When the revolved view is superimposed on the part,

the original lines of the part behind the section are

deleted.

(B) If the revolved section crosses lines of the view on

which it is to be revolved, then the view can be broken

for clarity.

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Removed section view is similar to revolved section with revolving the cross section 900. However, in this case the

cross section is then drawn adjacent to the orthographic view, not on it.

Removed sections are used when there is not enough room on the orthographic view for a revolved section.

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In the above-shown case the orthographic view is broken to show the section view, but there is also possible to have

a broken-out section.

A broken-out section is used when only a portion of the object needs to be sectioned. The following figure shows a

part with a portion removed or broken away:

A broken-out section is used instead of a half- or full-section view to save time, and a break line is drawn freehand

to represent the jagged edge of the break.

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Thin wall sections. Ribs, webs, spokes, gear teeth and other thin features are not section lined when the cutting plane

passes parallel to the feature. Adding section lines to these features would give the false impression that the part is

thicker than it really is.

The following figure shows a cutting plane that passes parallel to and through a web (SECTION B-B).

Leaving thin feature unsectioned only applies if the cutting plane passes parallel to the feature. If the cutting plane

passes perpendicular or crosswise to the feature (cutting plane A-A), section lines are added as shown in figure (C).

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Conclusions: the main principles of Placement of sectional views.

• Whenever practicable, and except for revolved sections, sectional views should be projected perpendicular to the

cutting plane and be placed in the normal position for third angle projection.

• They should never be shown in first angle projected position on a third angle projection drawing.

• When the preferred placement is not practical the sectional view may be removed to some other convenient

position on the drawing, but it must be clearly identified, usually by two capital letters, excluding I, O, Q, and

Z, and be labelled.

• Its orientation should not be changed, but if this becomes necessary, the number of degrees through which it is

revolved must be stated.

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